Adaptive radiations offer unique insight into how diversification is initiated in novel or changing environments but the value of such studies is often limited by incomplete or lacking information on the ancestral species. The threespine stickleback species complex is proving to be particularly valuable in enhancing our understanding of evolutionary processes because there is reason to believe a surrogate for the ancestral group is extant and representative of the oceanic form that gave rise to most post-glacial freshwater populations during the last ~12,000 years. If we are to maximize the value of this radiation a thorough understanding of the putative ancestor group is needed. This dissertation explores the degree of phenotypic variation in oceanic stickleback in Cook Inlet, AK as well as the relative contributions of genetic and plastic aspects shaping the phenotypic variation revealed.
Geometric morphometrics were used to describe shape differences in two oceanic forms of stickleback, anadromous and fully marine. These groups differ in shape along the same benthic-limnetic axis described within the freshwater derived populations in the same region. A common-garden rearing study revealed high levels of body shape plasticity in both groups as well as likely genetic influences maintaining important aspects of shape differences between their stocks of origin. Interestingly, plasticity related to the salinity of early rearing environment differed across types suggesting that there may be a flexible dual stem in the threespine stickleback radiation, a surprising result that has not been considered to date in any system to my knowledge.
Additionally, because life-history traits are intimately linked to reproductive success and thus fitness, differences in life-history strategies between these two oceanic types should reflect meaningful adaptive variation, whether plastic or strictly genetic based. Established methodologies in stickleback life-history studies were employed to assess phenotypic variation across populations, types, and years in many important traits (e.g., egg and clutch size, reproductive effort, allometric relationships between reproductive effort and female body size). Life-history strategies differed significantly across type and year. Generally, marine females exhibit greater reproductive investment and have larger and more numerous eggs per clutch. Anadromous populations experience an apparent reproductive cost to the migration to freshwater relative to their fully-marine counterpart. It’s unclear from these studies then where the fitness advantage to anadromy lies in the primitively oceanic species complex. However, important differences in mortality on the breeding grounds for adults and young as well as a possibly faster clutch production frequency in the anadromous lifestyle explains the apparent paradox in these data.
The finding of differences in genetic and plastic contributions to oceanic stickleback phenotypes body shape and life histories across two types in close geographic proximity which correlates with salinity regime suggest a flexible dual stem in the oceanic group(s). This could then influence evolution within the freshwater radiation. Thus, depending upon the freshwater populations (or watersheds) studied, the choice of representative oceanic type would need to be carefully considered. These data suggest that any near shore or inland sea areas within the stickleback oceanic distribution which experience a wide range of salinities is likely to show associated clinal variation in stickleback population reaction norms for (at least) body shape, life history strategies, and likely many other traits which are sensitive to salinity, such as genes involved in osmoregulation. Recent studies of Baltic and Sea of Japan oceanic stickleback further support this conclusion.
|Advisor:||Foster, Susan A.|
|Commitee:||Baker, John A., Livdahl, Todd P., Wund, Matt A.|
|School Location:||United States -- Massachusetts|
|Source:||DAI-B 77/08(E), Dissertation Abstracts International|
|Subjects:||Ecology, Evolution and Development|
|Keywords:||Adaptive radiation, Life history, Morphology, Plasticity, Salinity, Threespine stickleback|
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